Rats fixed with chronically indwelling electrodes for stimulation of the medial forebrain bundle of the lateral hypothalamus were the subjects used to collect preliminary. The rats were allowed to press, daily, a lever for the reDarding intracranial stimulation before, during, and after daily injections of morphine. Preliminary data indicated that with certain doses of morphine that rates of lever-pressing were increased. Also, after daily morphine injections, preliminary data indicated that the lower threshold of current intensity necessary to sustain lever- pressing was increased. These data suggest that an effect of addicitve agents might be to reorganize the medial forebrain bundle system in a way that makes the system more difficult to activate after exposure to daily morphine. If such a finding can be verified and if other addictive agents affect these systems similarly, then there is the strong possibility that these threshold shifts are basic processes of addictions. In colloquial terms, such shifts would make it more difficult for the addict to achieve pleasure, unless he had the addictive agent, and would make the addict more vulnerable to anxiety. Hence, the addict craves the addictive agent even beyond primary withdrawal. Because of the potential importance of our preliminary data, we want to test as completely as possible the reliability of the preliminary findings. Consequently, in a series of experiments, rats are to be tested for the after-effects of daily morphine injections on the lower threshold for intracranial reinforcement.